1. Field of the Invention
The present invention relates to a radio communication control device applied to, for example, a radio LAN (Local Area Network).
2. Description of the Related Art
In radio data transmission in conformity to standard IEEE802.11, a data communication control device receives transmission data in accordance with the standard defined by IEEE802.11. Then, when the data is judged to be addressed to the self station, the transmission of the data is started after a predetermined standby time period elapses. On the other hand, when it is judged that the data is addressed to some other station, the data transmission must be started after standing by for a predetermined time period different from the above. The start point of the stand-by period timer which initiates to time the stand-by period is set at the finish point of the last received data just before the data transmission. In this operation, it is essential to accurately find the finishing point of the last received data.
For determining the finishing point of the last received data, there are two generally known methods. The first method is of a type in which the intensity of the received electric filed of transmitted data is measured, and when the electric field is detected to become lower than a predetermined threshold value, that particular point is set to be the finishing point of the received data. The second method is of a type in which the medium occupation finishing time is calculated from the timing point when the data communication device receives the last data by its MAC (Media Access Control) layer, in consideration of the processing time in the PHY (Physical) layer.
FIG. 5 is a diagram showing an example of the structure which realizes the first method for measuring the intensity of the received electrical field. With reference to this structure, the first method will now be briefly discussed. That is, a signal received through an antenna 11 is supplied to a synchronizing unit/demodulating unit 13 via a sharing device 12 serving as a transmission/reception switching unit. The received signal is further supplied to an RSSI (Received Signal Strength Indication) processing unit 14. The RSSI processing unit 14 measures the intensity of the electric filed of a received signal, and supplies a control signal to a standby period timer 18 when the intensity of the electrical field thus measured is detected to be lower than a predetermined threshold value. The standby period timer 18 starts counting of the timer in accordance with the control signal and sets the standby period.
Note that a buffer circuit 15, a Viterbi decoder 16, a frame receiver 17, a stand-by period timer 18, a frame outputting unit 19 and a transmitting unit 20 will be described later.
FIG. 6 shows an example indicating the intensity of the electric field of a received signal. According to the first method for measuring the intensity of the electric field of a received signal, the point at which the electric field intensity of the signal shown in FIG. 6 becomes lower than a predetermined threshold value, for example, −40 dBm, is set to be the final point of the occupation of the medium by transmission data. When the electric field intensity of a received signal, measured by the RSSI processing unit 14 is equal to or less than this threshold value, the standby period timer 18 starts counting. However, as shown in FIG. 6, from the burst finishing point of the data part contained in the received signal to the point where the electric filed intensity of the received signal lowers to, for example, −50 dBm, it takes 4 μsec at maximum. This time period differs depending on the received signal, and therefore there results an error of 4 μsec at maximum.
Of standby time periods each between a frame and another frame, defined by IEEE802.11a, the minimum period type is called SIFS (Short Inter Frame Space), and the duration is 16 μsec at maximum. An error of 4 μsec indicates that there may result a maximum of 25% of error with respect to the SIFS. Further, in the case where there is a jamming wave in the electric field of the received signal, the electric field intensity of the received signal does not become lower than the predetermined threshold value, and therefore the operation of the timer cannot be initiated.
As described above, the first method of initiating the timer based on the measurement of the intensity of the electric field of the received signal entails a possible drawback of creating a significant error in determining the starting point of the timer operation, which requires a high precision, and therefore entails such a fatal drawback that once an error occurs, an abnormal communication process lasts forever in the worst case.
On the other hand, FIG. 7 illustrates an example of the structure which can realize the second method described above. In this figure, the same structural parts as those shown in FIG. 5 are designated by the same reference numerals. With reference to the structure shown in this figure, the second method will now be briefly described.
The synchronizing unit/demodulating unit 13 demodulates a signal of a required channel from the received signal, and convert it into a signal of a baseband. Further, the synchronizing unit/demodulating unit 13 detects I and Q signals (not shown) from the baseband signal, and demodulate a symbol of a received data columns from the I and Q signals. Thus, symbols are demodulated one after another, and thus demodulated symbols are supplied to the buffer circuit 15 successively. The Viterbi decoder 16 decodes the symbols stored in the buffer circuit 15. Output data from the Viterbi decoder 16 are supplied to the frame receiving unit 17 in the MAC layer. When the frame receiving unit 17 receives the final received data thus demodulated, the counting of the standby period timer 18 is started.
It should be noted that the second method calculates the medium occupation finishing time from the point when the final received data demodulated in the PHY layer is received by the frame receiving unit 17 of the MAC layer. However, the time required to demodulate received data by the PHY layer differs from a case to another depending on the length of data and the demodulating method. Therefore, it is difficult to calculate the starting point of the standby period in the MAC layer.
In general, the Viterbi decoder 16 decodes symbols in the unit of buffer size. However, the length of received data is variable. Therefore, in some cases, the number of symbols of received data becomes smaller than the size of the buffer circuit at the terminal end, and therefore there is a redundant space created in the buffer circuit 15.
FIG. 8 illustrates buffering operation in the buffer circuit 15.
As shown in this figure, when there is a redundant area results after the final symbol in the buffer circuit 15, the Viterbi decoder 16 is set in such a state that it cannot start the process. In order to avoid this, the PHY layer must carry out a process of inserting particular dummy data in the redundant space of the buffer circuit 15 in order to fill the space. Therefore, even in the same demodulating method, the time of buffering varies depending on the length of data.
Further, the amount of data contained in one symbol is different from one case to another depending on the demodulating method. Therefore, the buffering time varies depending also on the demodulating method.
Further, the length of data is variable and therefore in some cases, there is a gap space created at an end portion of one symbol having a certain length. In this case, the gap space must be filled.
Due to the above three factors combined, the process of the PHY layer becomes very complicated. Therefore, the second method, which calculates the medium occupation fishing time from the point when the MAC layer receives the demodulated final received data from the PHY layer, and sets the starting point of the timer then, becomes very much complicated in structure. As a result, the cost required for the hardware structure for the method is very expensive.
As described above, with the conventionally known first and second methods, it is very difficult to determine the starting point of the standby period timer, which is particularly significant for a radio communication device to control the timing of data transmission.
Under these circumstances, there has been a demand of a radio communication control device which can accurately determine the starting point of the standby period timer without having to provide a complex structure.